The invention relates to a color display device comprising an in-line electron gun for generating three electron beams, and a unit for deflecting the electron beams across the color selection electrode.
Such display devices are known.
A present aim is to make the outer surface of the display window flatter, so that the image represented by the color display device is perceived by the viewer as flat. However, an increase of the radius of curvature of the outer surface will lead to an increase of a number of problems. The radius of curvature of the inner surface of the display window and of the color selection electrode should increase, and, as the color selection electrode becomes flatter, the strength of the color selection electrode decreases and hence the sensitivity to doming and vibrations increases. An alternative solution to this problem would be to curve the inner surface of the display window more strongly than the outer surface. By virtue thereof, a color selection electrode having a relatively small radius of curvature can be used. As a result, doming and vibration problems are reduced, however other problems occur instead. The thickness of the display window is much larger at the edges than in the center. As a result, the weight of the display window increases and the intensity of the image decreases substantially towards the edges.
EP 0,421,523 discloses a color cathode ray tube with an in-line gun, a pin cushion correcting yoke and an eyebrow effect electronoptical distortion correction device comprising two pairs of coils, each pair having a coil on each outer electron beam side of the neck in the plane of the beams. The coil pairs are spaced apart along the Z-axis between the gun and the yoke and are driven by a sawtooth current having a bow-tie envelope synchronous with the raster scan to correct the dynamic, antisymmetrical eyebrow effect apparent as a purity defect on the raster.
WO 99/34392 describes a color display device comprising a color cathode ray tube including an in-line electron gun for generating three electron beams being located substantially within a plane extending in an X-direction of a rectangular X-Z coordinate system, a color selection electrode, deflecting means for deflecting the electron beams located at a deflection plane, and first and second influencing means to dynamically influence the convergence of the electron beams, to decrease a distance between the electron beams at a location of the deflection plane. WO 99/34392 describes many different ways in which the influencing means may be embodied. One of the possibilities mentioned is to integrate the second influencing means with the deflection unit. The second influencing means may be embodied as four coils, which generate a magnetic quadrupole field.
It is an object of the invention to provide an improved color display device.
To this end, the invention provides a color display device, a deflection unit and a method of manufacturing such a color display device in which convergence of three electron beams from an in-line electron gun is dynamically influenced by a toroidal winding for producing a magnetic quadrupole field. Such a toroidal winding has a winding density distribution N(xcfx86) which varies as the cosine of twice the angle xcfx86 enclosed by the X-direction and a line between an element of the coil and the center, where the X-direction is substantially transverse to the three electron beams and lies in a plane in which the three electron beams are located substantially. According to the invention the winding density has a finite non-zero value at the positions where cos (2xcfx86)=0. Preferably the winding density at xcfx86=0 is zero.